1. Field of the Invention
The invention relates to the determination of the flight parameters of flying vehicles or to other fields of science and technology which deal with flows of liquid or gas.
The measurement of flight parameters is one of the most important tasks of the aeromechanics and aerodynamics of flying vehicles (FVs). At the present time, in order to measure the flight (flow) parameters use is made of Pitot-static tubes (PSTs) which are, frequently, mounted directly on the fuselage of the aircraft or on the body of any other flying vehicle, and these PSTs actually measure parameters of the local flow, which is close to laminar. As a rule, several such PSTs are mounted on the flying vehicle and measure the local flow parameters. The true flight parameters are determined on the basis of preliminary calibrations.
2. Description of the Related Art
A Pitot-static tube mounted on the body or fuselage of an FV is known from WO 94/02858. The known PST has a cylindrical tube mounted on a strut having curved leading and trailing edges which approach one another as the tube is neared from the base of the strut. The leading edge of the strut can be rounded off. The pitot-static tube has an orifice in the nose part of the tube for sensing the total pressure, and an orifice for sensing the static pressure at a certain distance from the nose of the tube. The tube has a heater for preventing the formation of ice. However, this Pitot-static tube cannot be applied for determining the angle of attack, since it lacks orifices for sensing pressure with the aid of which the angle of attack can be measured. In fact, as follows from the abovementioned patent, this tube is not intended for these purposes. Moreover, the tapering of the strut, seen from the side, as the tube is approached leads, in conjunction with maintaining the internal volumes required for installing airways and heaters, to a marked increase in the relative thickness of the profiles of the transverse cross-sections of the strut. This leads, in turn, in the case of high subsonic speeds (Mach numbers of M=0.8-0.9) to the earlier appearance of local pressure shocks and a marked increase in the shock drag of such a Pitot-static tube.
A fuselage Pitot-static tube according to U.S. Pat. No. 4,615,213 is known for determining the flight (flow) parametersxe2x80x94angle of attack, total pressure Po and static pressure Ps and, consequently, also the Mach number M; it is an elongated axisymmetric body having a head part in the form of a hemisphere with groups of orifices on the axisymmetric body for measuring pressures by means of which the flight (flow) parameters are determined with the aid of calibrations. At the same time, the orifices for measuring the pressures by means of which the total pressure and angle of attack are determined are arranged on the hemispherical head part, while the orifices for measuring the static pressure are arranged on the lateral (cylindrical) surface of the axisymmetric body. For the purpose of mounting on the fuselage or body of the flying vehicle, this PST has a strut, the profile of which has a lens-shaped transverse cross-section. The given PST has the following disadvantages:
a complicated design;
increased overall dimensions of the axisymmetric body;
increased aerodynamic drag in subsonic flight regimes;
increased required power for the heater of the anti-icing system;
increased design weight;
increased sensitivity of the total pressure, measured with the aid of the central orifice on the spherical head part, to variation in angle of attack, which leads to additional errors in measurement of the total pressure; such a dependence of the total pressure on the angle of attack for a range of FVs is unacceptable.
The closest of the known technical solutions is disclosed in U.S. Pat. No. 4,378,696 for determining flight (flow) parametersxe2x80x94angle of attack, total pressure Po and static pressure Ps, and thus the Mach number M, which is an elongated axisymmetric body with a conical or ogival head part where an orifice is arranged for sensing total pressure, and which merges into a circular cylinder on whose surface orifices are arranged for sensing static pressure. Later, this cylindrical surface merges into a conical one, on which orifices are arranged for sensing pressure for which the angle of attack is set up correspondingly, and then merges again into the cylindrical surface. For the purpose of being mounted on the fuselage or the body of an FV, the tube has a strut whose cross-section has lens-shaped profile. The given PST has the following disadvantages:
complicated design;
increased overall dimensions;
increased aerodynamic drag in subsonic flight regimes;
increased required power for the heating anti-icing system;
increased design weight;
low sensitivity of pressures, measured in orifices arranged on a conical part (and intended for determining xcex1), to the angle of attack, which leads to increased errors in determining the angle of attack. This is caused by the following factors:
1. As in the case described above, the given PST has an increased mid-section of the axisymmetric body. Moreover, the increased dimension of the mid-section is caused in the given instance by two circumstances. The first is that the cylindrical part of the axisymmetric body merges into a conical one on which orifices are arranged for sensing the pressure by which the angle of attack is determined. In order to increase a little the sensitivity of the pressure sensed by means of these orifices of the angle of attack, the angle of taper must be sufficiently large to lead to the necessity of increasing significantly the diameter of the axisymmetric body behind the given conical part.
The second circumstance is bound up with the fact that although groups of orifices for measuring pressure, which are used to determine total pressure, static pressure and angle of attack, are dispersed in the given configuration, they are all situated on the same axisymmetric body. There is a need to arrange inside the latter airways emerging from all the indicated groups of orifices, a static pressure chamber and also tubular electric heaters for the anti-icing system. The diameters of the airways and the TEHs cannot be less than a certain minimum values which for the airways are determined by the magnitude of the hydrodynamic lag and for the TEHs by the limiting values of the heat flux density and the temperature of the surface of the heaters. The result is a high design saturation, that is to say a complicated design of the axisymmetric body of the PST.
The circumstances indicated lead to an increase in the area of the mid-section, and consequently to an increase in the design weight, aerodynamic drag and power of the anti-icing system.
It should also be pointed out that transition from the cylindrical part to the conical one, and then again to the cylindrical one, can lead to separation of the flow behind the conical part and to an earlier appearance (in terms of the Mach number) of local pressure shocks. This, in turn, must lead to an increase in the aerodynamic drag. Moreover, the increased diameter of the axisymmetric body and the non-optimum form of its tail part in conjunction with the strut also lends [sic] an unfavourable aerodynamic interference (separation of the flow and earlier appearance of pressure shocks) in the area of the joint of the contracting tail part of the axisymmetric body of the PST behind the line of maximum thickness of the lens-shaped aerodynamic profile of the strut. This also leads to a certain increase in the aerodynamic drag of such a PST.
It may also be noted that the presence of a conical part on the axisymmetric body of a PST leads to the realization of additional support on the cylindrical part lying in front, where the orifices for measuring static pressure are arranged. As a result, the precise determination (without the introduction of corrections) of static pressure requires that the orifices for sensing it must be sufficiently far from this conical part. This leads to the need to increase the length of the axisymmetric body, and also leads to a certain additional increase in the design weight, and requires additional power in the electric heating anti-icing system.
2. The lens-shaped profile of the strut is not optimum from the point of view of the aerodynamic drag in subsonic flight regimes. This leads to a substantial increase in the aerodynamic drag of the strut of the PST in subsonic flight regimes. Moreover, at very low Mach numbers the increase in aerodynamic drag is caused by separation from the sharp leading edge of the strut with the lens-shaped profile, which always takes place, since the leading edge is sharp, at local angles of attack other than zero. Since the lens-shaped profile is not optimum from the point of view of shock drag, at high subsonic speeds (M=0.8-0.9) the aerodynamic drag of such a PST is also increased very greatly. Although sweeping the leading and trailing edges of the PST strut postpones the sharp increase in shock drag, it leads to an increase given the same stagger of the axisymmetric PST body with respect to the fuselage, that is to say given the same strut height, overall dimensions, weight and volume of design and, consequently, also the required power of the anti-icing system.
3. Electric heaters arranged inside the PST strut for preventing the formation of ice on its leading edge, and thereby preventing the influence of this ice on measurement of pressure on the axisymmetric body, are insufficiently efficient in use in the sense that they heat the strut on which no orifices are arranged for measuring pressure. This lead to a substantial increase in weight and required electric power.
4. The lens-shaped profile of the strut is not optimum from the point of view of:
a predisposition to the formation of ice;
the design of the anti-icing system.
This leads to a substantial increase in the required power of the anti-icing system of the actual strut of the PST, which is caused by the following circumstances.
As is known (compare, for example, Bragg M. B., Gregorek G. M., Lee J. D., Airfoil Aerodynamic in Icing Conditions. J. Aircraft, vol. 23, No. 1., 1986), the formation of ice on a flying vehicle during flight in the atmosphere takes place, first and foremost, in areas adjoining points where the flow is decelerated and in areas of separation of the flow from the leading edge (for example, the wing). At the same time, it is noted that sharp leading edges of the wing are frequently more strongly subjected to the formation of ice than rounded-off ones, since a stream with a separation of flow always forms on them in the case of angles of attack other than zero. Such an area of the strut of a PST is an area adjoining its leading edge. Since the lens-shaped profile of the strut has a sharp leading edge, a stream with separation of flow from the front edge can form even in the case of small angles of attack, and this can lead to an intense formation of ice.
Since the TEHs of the anti-icing system are quite bulky and occupy substantial volumes, they cannot be arranged inside the strut in the immediate vicinity of the sharp edge of the lens-shaped profile of the strut. As a result, the TEHs on such a strut are arranged near the line of maximum thickness of the profile of the strut, while the heating of the critical zone, where ice actually formsxe2x80x94the area near the leading edge of the strut of the PST, results from heat transfer directly over the structure of the strut, from the line of maximum thickness to the leading edge. Although struts of modern PSTs are made from materials which conduct heat very well and are very expensive (for example, from nickel alloys), very large, inefficient heat losses reaching an estimated 50% are inherent in such a design.
Thus, the low coefficient of use for the energy supplied to the electric heaters is characteristic of such a design of a PST. However, since they are quite bulky this leads to a significant increase in the design weight.
5. The difference in the pressures measured at the conical part of the PST has a comparatively weak sensitivity to the change in the angle of attack, and this leads to increased errors in the measurement of the angle of attack. The increase in the aperture of the cone somewhat exceeds the sensitivity, but this leads to an increase in the diameter of the mid-section of the axisymmetric body of a PST, which entails an increase in the design weight, the aerodynamic drag and the required power of the anti-icing system. There are bodies where this sensitivity is substantially higher.
The nearest of the known symmetrical aerodynamic profiles suitable for use on the strut of a PST are the profiles of the NACA-00XX series (where XX is the relative thickness of the profile in per cent); the disadvantage of these profiles resides in the rapid growth in shock drag at high transonic numbers M. This is caused by the high degree of the diffusor effect of the profiled in the zone located behind the maximum thickness of the profile, which causes the earlier appearance of the pressure shock, as well as an increase in its intensity.
The objects of the invention are:
simplification of the design,
reduction in the overall dimensions,
reduction in the aerodynamic drag of the axisymmetric body of the PST,
reduction in the aerodynamic drag of the strut of the PST by developing the contour of the symmetrical aerodynamic profile for the strut of the PST which has a higher critical Mach number in the operating range of numbers M=0-0.85 by comparison with known symmetrical aerodynamic profiles, in particular with a lens-shaped profile (composed of arcs of a circle) or profiles of a series NACA-00XX for identical values of the relative thickness,
reduction in the required power of the heating anti-icing system,
reduction in design weight,
an increase in the accuracy of determination of the angle of attack on PSTs intended for subsonic non-manoeuvred flying vehicles.
The technical result is achieved by virtue of the fact that the fuselage Pitot-static tube comprising three groups of orifices for determining the total pressure, static pressure and angle of attack, and an axisymmetric body and strut for mounting an anti-icing system having, arranged between them, airways and electric heating elements, is constructed in such a way that the orifices for determining the angle of attack are arranged on the strut, whose cross-section is constructed in the form of a subsonic aerodynamic profile with a rounded-off nose or a tapered nose, and lie at some distance from the nose of the profile up to its maximum thickness.
For the purpose of a greater reduction in the aerodynamic drag of the fuselage sensor, the tail part of the axisymmetric body may terminate with and may be smoothly joined to the aerodynamic profile of the strut in the region of its maximum relative thickness, while for the purpose of reducing the aerodynamic drag at high subsonic speeds the tail part of the axisymmetric body can have a taper and a base cut, and for this purpose the trailing edge of the aerodynamic profile of the strut can also have a base cut.
In order to compensate for the influence of the fuselage or support of the strut on the measured static pressure, the axisymmetric body may have on the cylindrical part a swelling on which the orifices for measuring the static pressure are arranged.
The aerodynamic profile of the strut can be constructed asymmetrically for the purpose of additionally increasing the sensitivity of the variation in pressure to the angle of attack and of extending the range of the angle of attack.
For the purpose of an even greater reduction in the required power of the anti-icing system, the electric heating elements of the anti-icing system may be displaced towards the leading edge of the strut.
A simplification of the design of the axisymmetric body and a substantial reduction in its diameter are achieved by virtue of the fact that the orifices for measuring pressure which are used to measure the angle of attack, are arranged not on the axisymmetric body but on the strut of a PST. Since the design weight is proportional to the cube of its linear dimensions, given the same length of the axisymmetric body, reduction in its weight will be determined as the product of a certain coefficient and the difference of the squares of the diameter of the axisymmetric body of the PST prototype and the proposed PST. Since the aerodynamic drag of the axisymmetric body given a zero angle of attack of the PST is proportional to the area of its mid-section, the reduction in the aerodynamic drag of the PST were it to have the same form as the PST prototype would also be proportional to the difference of the squares of the diameters of the axisymmetric body of the PST prototype and the proposed PST. However, since the form of the axisymmetric body of the proposed PST does not have additional steps (conical step with a subsequent swelling) as in the PST prototype, there will be no separation of flow on it nor any appearance of pressure shocks behind the conical step. Thus, the reduction in the aerodynamic drag will be even larger. Since the required power for heating the axisymmetric body is proportional to the area of the surface of revolution of the axisymmetric body, reduction in the power for heating the proposed PST by comparison with the PST prototype (given the same temperature of their surface) is proportional to the difference between the diameters of the axisymmetric body of the PST prototype and the proposed PST. Moreover, reduction in the required power of the heating system leads to a reduced weight of the TEHs.
The strut of the PST can be constructed in such away that its cross-sections have the form of a subsonic aerodynamic profile with a chord of length B, a rounded-off leading edge and a sharpened or blunted trailing edge interconnected by the smooth lines of the contours of the upper and lower surfaces. The lower part of the contour of the profile is symmetrical to the upper part relative to the profile chord. The leading edge of the profile has a radius of curvature Rc which is in the range of Rc=0.030*B-0.034*B, in that the maximum relative thickness of the profile C is in the range of C=0.146-0.156 and is arranged at a distance of X=0.3*B-0.6*B, measured from the leading edge along its chord. The radius of curvature of the upper part of the profile contour increases smoothly along the profile chord with increasing distance X from the rounded-off leading edge up to the values of X=(0.3-0.6)*B for which part of the contour has a virtually rectilinear form up to the values of R=5.5*B-15.*B, it being the case that distance Yu, measured from the profile chord along the normal to it upwards to the upper part of the profile contour, increases smoothly to its maximum value of Yumax=0.074*B=0.078*B. The distance Yu further decreases smoothly along the direction towards the trailing edge, the radius of curvature firstly decreases smoothly down to the values of R=0.6*B-1.*B for X=0.82*B-0.95*B, and thereafter it increases smoothly up to the values of X=0.92*B-0.95*B, where the convex part of the contour is joined smoothly to its concave tail part and, further, the radius of curvature of the concave part of the contour decreases smoothly, reaching at the trailing edge of the profile values of R=0.05*B-0.5*B, the angle between the tangent to the profile contour and the chord of the profile at its trailing edge being 3-6 degrees for X=B. As the results of the calculations showed, the selected form of the contour and the distribution of curvature along its chord permits a substantial reduction in the shock drag of the profile both in comparison with the profile of the PST prototype (lens-shaped) and in comparison with the profile prototype (NACA 0015). Since when producing flying vehicles it is possible in a real design to realize theoretical coordinates of the profile contour only with a certain limited accuracy determined by the aggregate deviations of the actual coordinates of the points of the profile contour from the theoretical ones, which deviations accumulate at all stages of design and manufacture, the coordinates of the profile contour corresponding to the given invention must be in the interval of values given by Table 1:
In practice, additional design and aerodynamic requirements frequently arise, which amount to comparatively small changes in the relative thickness of the profile and are expressed in the fact that the dimensionless ordinates, referred to its chord, of the contours of the upper Yu/B and lower Yl/B surfaces differ from corresponding dimensionless ordinates of the base profile of the original relative thickness by equal constant numerical factors.
The transition to a different relative thickness for the profile by the given invention is possible by multiplying the ordinate of its contour by equal constant numerical factors Ku for the upper and Kl for the lower parts of the contour, the radii of curvature of the leading edge of the profile over its upper and lower surfaces varying in a fashion proportional to the square of the coefficients, and the numerical values of these factors having to be in the ranges of 0.8 less than Ku less than 1.07 and 0.8 less than Kl less than 1.07. Owing to the fact that the strut of the PST is constructed in such a way that its cross-sections have the form of a subsonic aerodynamic profile with a rounded-off nose, and not of a lens-shaped profile, as on the PST prototype, its aerodynamic drag can, as indicated by calculations, be reduced by 2-2.5 times in the case of the number M=0.8-0.9.
It is known that the formation of ice during flight in the atmosphere chiefly affects areas of flow deceleration or areas where a separation of flow is formed. Owing to the occurrence on them of streams with flow separation, sharp leading edges are frequently more subject to the formation of ice than are rounded-off ones. Since, by contrast with the lens-shaped profile, where even at small angles of attack a stream is formed with separation of flow from the leading edge, there is no separation of flow at small angles on a subsonic aerodynamic profile with a rounded-off nose, the strut of the proposed PST is less subject to the formation of ice than the strut of the PST prototype. Moreover, in the case of the strut of the PST prototype, because of the fact that it has a cross-section in the form of a lens-shaped profile, it is difficult or virtually impossible to arrange the electric heaters of the anti-icing system immediately next to the nose of the profile, since the volumes required for this are not present inside. Consequently, the electric heaters for such a PST are arranged not in the nose itself (which is most subject to the formation of ice) but near the centre of the profile. As a result, heating of the nose is due to heat transfer along the strut, and this causes large power losses (estimated at up to 50%). In the proposed PST, the radius of the nose of the subsonic aerodynamic profile can be made sufficiently large to permit the electric heaters to be arranged directly in the nose of the strut, and thereby to reduce power losses by 25-30%.
Since the critical Mach number (at which pressure shocks occur) on the subsonic aerodynamic profile with a rounded-off nose, in particular on the profile according to the given invention, can be substantially lower than on a lens-shaped one, the sweep angle of the strut of the PST designed for flights with M=0.8-0.9 can be made substantially smaller for the proposed PST than for the strut of the PST prototype. As estimates indicate, for the same height of the struts and profile chord this yields a reduction in the length of the PST and a gain in design weight by 10-15%.
Since the sensitivity to variation in the angle of attack of pressures measured on a subsonic aerodynamic profile with a rounded-off nose is substantially higher than on a cone, the error in measurement of the angle of attack is substantially lower for the proposed PST than for the PST prototype.
The trailing edge of the aerodynamic profile of the section of the strut can be constructed with a base cut for the purpose of additionally reducing the shock drag at numbers of M=0.8-0.9 involving, in terms of Mach number, occurrences of pressure shocks and their displacement to the tail of the profile owing to the lesser diffusor effect of the profile behind the point of its maximum thickness. Constructing the tail part of the axisymmetric body with a taper and base cut also permits, in a fashion analogous to the aerodynamic profile, a reduction in the shock drag of the PST. If the tail part of the axisymmetric body starts to taper in the area of the maximum thickness of the profile of the strut, a strong diffuser which leads to an earlier occurrence of local pressure shocks and an increase in aerodynamic drag is formed in the area of the joint of the tail part of the body and the strut. In the case when the axisymmetric body is constructed in such a way that its tail part terminates with and is smoothly joined to the aerodynamic profile of a strut in the area of its maximum relative thickness, there is an improvement in the interference of the axisymmetric body and strut, and there is an additional substantial decrease in the aerodynamic drag of the PST because of the absence of an additional diffuser. Owing to the fact that the aerodynamic profile of the strut can be constructed asymmetrically, there is an increase in the sensitivity of pressure to the angle of attack, and it is thereby possible additionally to increase the accuracy of measurement of the angle of attack; moreover, the range of the angle of attack can be widened owing to the asymmetry of the profile. To compensate the effect of deceleration from the strut on the measurement of static pressure, the axisymmetric body can have on the cylindrical part a swelling on which orifices for measuring static pressure are arranged. Owing to the acceleration of the flow on this swelling, it is possible to find an area where the deceleration from the strut is compensated for by this acceleration and, consequently, the precise static pressure can be selected from the indicated orifices. Because of the displacement of the electric heating elements towards the leading edge of the strut, there is a substantial reduction in the inefficient thermal losses by comparison with the PST prototype, and a reduction in the required power for heating.